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A protist (/ ˈ p r oʊ t ɪ s t / PROH-tist) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus.Protists do not form a natural group, or clade, but are a polyphyletic grouping of several independent clades that evolved from the last eukaryotic common ancestor.
All eukaryotes apart from animals, plants and fungi are considered protists. [1] Its field of study therefore overlaps with the more traditional disciplines of phycology , mycology , and protozoology , just as protists embrace mostly unicellular organisms described as algae , some organisms regarded previously as primitive fungi , and protozoa ...
Many protists have protective shells or tests, [139] usually made from calcium carbonate (chalk) or silica (glass). Protists are mostly single-celled and microscopic. Their shells are often tough mineralised forms that resist degradation, and can survive the death of the protist as a microfossil. Although protists are very small, they are ...
Green algae have a "stigma" located in the outermost portion of the chloroplast, directly underneath the two chloroplast membranes. The stigma is made of tens to several hundreds of lipid globules, which often form hexagonal arrays and can be arranged in one or more rows.
Chloroplasts have many similarities with cyanobacteria, including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction center. [ 204 ] [ 205 ] The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis ) by early eukaryotic cells to form the first plant cells.
Indeed, the chloroplast of the chromists is located in the lumen of the endoplasmic reticulum instead of in the cytosol. Moreover, only chromists contain chlorophyll c. Since then, many non-photosynthetic phyla of protists, thought to have secondarily lost their chloroplasts, were integrated into the kingdom Chromista.
Plastid types in algae and protists include: Chloroplasts: found in green algae (plants) and other organisms that derived their genomes from green algae. Muroplasts: also known as cyanoplasts or cyanelles, the plastids of glaucophyte algae are similar to plant chloroplasts, excepting they have a peptidoglycan cell wall that is similar to that ...
For example, the algae Euglena and Dinobryon have chloroplasts for photosynthesis, like plants, but can also feed on organic matter and are motile, like animals. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these ...